1. Field of the Invention
The present invention relates to an aligning, device provided with a sensor for detecting the position of a substrate with a resolving power of the order of a nanometer (nm), and more particularly to such aligning device adapted for use in an lo exposure apparatus for semiconductor device manufacture, such as a stepper or an aligner.
2. Related Background Art
In recent photolithographic processes for semiconductor device manufacture, there are widely employed reduction projection exposure apparatus of step-and-repeat type (steppers) for transferring reticle patterns onto a wafer with a high resolving power. With the progress in the level of integration in the semiconductor devices, there have been developed the use of shorter wavelengths in the exposing light and lenses of larger numerical aperture (N.A.) for use in such steppers, and a sub-micron line width (0.5-0.7 .mu.m) has been recently resolved on the wafer. For transferring such fine pattern, there is required a precision of alignment matching such resolving power, and there has been conceived an improvement in the aligning accuracy for example through an improvement in the detecting resolving power of the alignment system.
Such alignment system of high resolving power, for example disclosed in the U.S. Pat. No. 4,710,026, consists of irradiating a one-dimensional diffraction grating mark formed on a wafer with coherent parallel beams from two directions to form one-dimensional interference fringes on said diffraction grating mark, and photoelectrically detecting the intensity of diffracted light (interference light) generated from said diffraction grating mark by the illumination with said interference fringes.
The disclosed system can be realized in a heterodyne method in which the parallel beams from two directions are given a predetermined frequency difference, or a homodyne method in which said beams have no difference in frequency. In said homodyne method, stationary interference fringes are formed parallel to the diffraction grating mark. For position detection, the diffraction grating mark (object) has to be moved slightly in the direction of pitch, and the mark position is determined with reference to the interference fringes. On the other hand, in the heterodyne method, the interference fringes move rapidly with the beat frequency in the direction of pitch, so that the mark position cannot be determined with reference to the interference fringes but can only be determined with reference to a time factor (phase difference) related to the high-speed movement of the interference fringes.
More specifically, the heterodyne method is to detect a positional aberration within .+-.P/4, wherein P is the pitch of grating, by determining the phase difference (within .+-.180.degree.) between a photoelectric signal (optical beat signal) obtained by intensity modulation of the interference light from the diffraction grating mark of the wafer with the beat frequency of the frequency difference and an optical beat signal of reference interference light separately prepared from two light beams. If the grating pitch P is 2 .mu.m (line and space of 1 .mu.m) and the resolving power for the phase difference measurement is of the order of 0.5.degree., the resolving power for the measurement of positional aberration is (P/4).multidot.(0.5/180).congruent.0.0014 .mu.m. As such method can detect the mark position with an extremely high resolving power, it is expected to achieve an alignment accuracy higher by more than one order than the conventional method for mark position detection.
However, such alignment system may result in a loss of alignment accuracy and become unable to fully exhibit the advantage of high resolving power unless the crossing angle of two laser beams is exactly adjusted so as to satisfy a relation P=m.multidot.P' (m=1, 2, . . .) between the grating pitch P and the pitch P' of the interference fringes, and the interference fringes and the grating are made mutually exactly parallel so that the rotational error of a crossing line between a plane containing principal rays of two laser beams and the wafer surface with respect to the direction of arrangement of grating is substantially zero. In conventional practice, therefore, the pitch P' of the interference fringes is exactly set with respect to the grating pitch P so as to satisfy the above-mentioned relation by photoelectrically detecting the interference light from the grating mark while varying the crossing angle of two laser beams, and regulating said crossing angle so as to maximize the intensity of said interference light. On the other hand, the rotational error of the crossing line between the plane containing the principal rays of two laser beams and the wafer surface is substantially brought to zero, by photoelectrically detecting the interference light from the diffraction grating mark and mutually rotating the diffraction grating mark (wafer) and the interference fringes so as to maximize the intensity of the interference light.
However, in such conventional technology, the crossing angle of two laser beams and the rotational error are measured and regulated by the detection of maximum value of the diffracted light intensity (namely voltage of the optical beat signal), namely by the peak shooting method. Such peak shooting method is associated with an essential drawback that the rate (sensitivity) of signal variation is zero at the maximum value. Also the calculation of the crossing angle or the rotational error by monitoring of electric levels is apt to be influenced by noises, and is unable to provide satisfactory accuracy of measurement.